In MIMO technology, the capacity of a radio network is improved by providing a plurality of antennas and carrying out data transmission via diversity or a multiplexing scheme. In a conventional communication system, normally a plurality of antennas are arranged in a one-dimensional array in a radio base station, and a plurality of user terminals (UEs) are able to be distinguished in the horizontal dimension. For example, in a radio base station, four emission antennas may be provided in order to simultaneously carry out communication with a plurality of UEs, and the four emission antennas may be arranged in a 4×1 one-dimensional array.
However, for example, in a three-dimensional cell environment of macrocells and small cells, etc., the UEs are positioned at different heights; for example, a plurality of UEs distributed on different floors of a building. In such a case, it is difficult for a pre-existing linear array, in which a plurality of UEs are separated only in the horizontal dimension, to correctly separate the plurality of UEs that are positioned at different heights. In view of this issue, a two-dimensional antenna array has been proposed, thereby enabling enhancement in spatial diversity in the vertical dimension.
A conventional method of generating a precoding matrix relates to a one-dimensional antenna array. For example, the latest 3GPP specifications assume the plurality of antennas provided in the radio base station to be distributed in a uniform linear array (ULA), and codebooks for the precoding matrix have been determined based on this assumption. In addition, different one-dimensional arrays have been proposed, and precoding matrix codebooks have been respectively proposed for these different one-dimensional arrays. For example, in Rel. 8 of the 3GPP specifications, a precoding matrix codebook has been proposed in regard to a 2×1 one-dimensional antenna array; in Rel. 10, a precoding matrix codebook has been proposed in regard to a 8×1 one-dimensional antenna array; and in Rel. 12, a precoding matrix codebook has been proposed in regard to a 4×1 one-dimensional antenna array. However, the configuration schemes of the precoding matrix codebooks for these different one-dimensional antenna arrays are not the same, thereby being disadvantageous in regard to flexibility in arranging antenna arrays.